System and method for collecting 3He gas from heavy water nuclear reactors

ABSTRACT

A method of collecting 3He from a nuclear reactor may include the steps of a) providing heavy water at least part of which is exposed to a neutron flux of the reactor, b) providing a cover gas in fluid communication with the heavy water, c) operating the nuclear reactor whereby thermal neutron activation of deuterium in the heavy water produces tritium (3H) and at least some of the tritium produces 3He gas by β− decay and at least a portion of the 3He gas escapes from the heavy water and mixes with the cover gas, d) extracting an outlet gas stream, the outlet gas stream comprising a mixture of the cover gas and the 3He gas and e) separating the 3He gas from the outlet gas stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/852,216, filed Mar. 28, 2013, which claims the benefit of U.S.Provisional Patent Application No. 61/617,802, filed Mar. 30, 2012, andthe entire contents of which are hereby incorporated by reference.

FIELD

The present subject matter of the teachings described herein relatesgenerally to a method and system for collecting ³He gas from heavy watermoderated and/or cooled nuclear reactors.

BACKGROUND

³He is an isotope of helium with applications in many differentindustries. ³He can be formed by beta decay of tritium.

One known source of ³He gas is the decay tritium in nuclear weapons.Another source of tritium is the irradiation of tritium producingburnable absorber rods (TPBARs) within light water nuclear reactors.

Another source of tritium is heavy water nuclear reactors. Heavy waterincludes deuterium. Heavy water reactors, for example reactors that useheavy water as a moderator, coolant or both, may produce tritium as aresult of thermal neutron activation of the deuterium in the heavywater. The heavy water can be detritiated, the tritium can be collectedand ³He may be obtained as the tritium decays.

Demand for ³He may exceed the supply of ³He from known production and/orcollection methods. ³He may be commercially valuable. Therefore, thereremains a need for an alternative apparatus and/or system for directlycollecting ³He.

SUMMARY

This summary is intended to introduce the reader to the more detaileddescription that follows and not to limit or define any claimed or asyet unclaimed invention. One or more inventions may reside in anycombination or sub-combination of the elements or process stepsdisclosed in any part of this document including its claims and figures.

In accordance with one broad aspect of the teachings described herein, amethod of collecting ³He from a nuclear reactor may include the steps ofa) providing heavy water at least part of which is exposed to a neutronflux of the reactor, b) providing a cover gas in fluid communicationwith the heavy water, c) operating the nuclear reactor whereby thermalneutron activation of deuterium in the heavy water produces tritium (³H)and at least some of the tritium produces ³He gas by β⁻ decay and atleast a portion of the 3He gas escapes from the heavy water and mixeswith the cover gas, d) extracting an outlet gas stream, the outlet gasstream comprising a mixture of the cover gas and the 3He gas and e)separating the 3He gas from the outlet gas stream.

The method may also include outputting a ³He gas stream for furtherprocessing and may include treating the outlet gas stream to provide atreated cover gas stream.

The method may include mixing at least a portion of the treated covergas stream into the cover gas in fluid communication with the heavywater.

The step of extracting the outlet gas stream may be performed whilenuclear reactor is operating and the outlet gas stream may be extractedas a generally continuous stream while nuclear reactor is operating.

The step of separating the ³He gas from the outlet gas stream may be anon-line process that is performed while the nuclear reactor isoperating.

The step of separating the ³He gas from the outlet gas stream mayinclude at least one of a thermal diffusion process, a fractionaldiffusion process, a heat flush process, a superleak process and adifferential absorption process.

The cover gas may contact the heavy water at a free surface interface.

A method of collecting ³He from a nuclear reactor may include the stepsof a) providing heavy water at least part of which is exposed to aneutron flux of the reactor, b) operating the nuclear reactor wherebythermal neutron activation of deuterium in the heavy water producestritium (³H) and at least some of the tritium produces ³He gas by β⁻decay and at least a portion of the ³He gas escapes from the heavywater, c) extracting an outlet gas including the ³He gas, and d)optionally, separating the ³He gas from any other gas in the outlet gasstream.

According to another broad aspect of the teachings described herein, asystem for collecting ³He may include a nuclear reactor having a vesselcontaining a heavy water and having a cover gas head space containing acover gas above the heavy water. The reactor may have a gas outlet incommunication with the cover gas head space. Operation of the nuclearreactor may result in thermal neutron activation of deuterium in theheavy water to produce tritium (³H) and at least some of the tritium mayundergo β⁻ decay to produce ³He gas that mixes with the cover gas. A gasextraction passage may be fluidly connected to the gas outlet of thevessel to extract a gas outlet stream through the gas outlet. The gasoutlet stream may include the cover gas and the ³He gas mixed with thecover gas. A ³He separation apparatus may be fluidly connected to thegas extraction passage downstream gas outlet and may be operable toreceive the gas outlet stream and separate the ³He gas from the covergas.

A gas inlet may be provided in the vessel and in communication with thecover gas head space. A cover gas supply passage may be coupled to thegas inlet of the vessel to supply the cover gas to the cover gas headspace.

The ³He separation apparatus may include a ³He outlet to output aseparated ³He gas stream and a separate treated cover gas outlet tooutput a treated cover gas stream.

The treated cover gas outlet of the ³He separation apparatus may befluidly connected to the cover gas supply passage to re-introduce atleast a portion of the treated cover gas stream into the cover gas headspace.

The gas outlet stream may be extractable as a generally continuous gasstream while the nuclear reactor is in operation.

The cover gas provided above the heavy water may consist essentially of⁴He.

The ³He separation apparatus may include at least one of a thermaldiffusion apparatus, a fractional diffusion apparatus, a heat flushapparatus, a superleak apparatus and a differential absorptionapparatus.

According to yet another broad aspect of the teachings described hereina moderator cover gas system for use with a nuclear reactor having avessel containing heavy water may include a cover gas supply passagehaving a gas outlet connectable to a gas inlet on the vessel to supply acover gas into the vessel. A gas extraction passage may have a gas inletconnectable to a gas outlet on the vessel to extract an outlet gasstream from within the vessel. The outlet gas stream may include amixture of at least the cover gas and ³He gas. A gas separationapparatus may be connected to the cover gas flow passage downstream fromthe gas outlet on the vessel and operable to separate the ³He gas fromthe outlet gas stream.

A fresh cover gas source may be fluidly connected to the cover gassupply passage to introduce cover gas consisting essentially of ⁴He intothe interior of the vessel.

The gas separation apparatus may include a first outlet to output theseparated ³He gas and a second outlet to output a treated cover gasstream. The second outlet may be fluidly connected to the cover gassupply passage to feed at least a portion of the treated cover gasstream into the cover gas supply passage.

DRAWINGS

The drawings included herewith are for illustrating various examples ofarticles, methods, and apparatuses of the teaching of the presentspecification and are not intended to limit the scope of what is taughtin any way.

In the drawings:

FIG. 1 is a schematic representation of a heavy water nuclear reactor;

FIG. 2 is a schematic representation of another example of heavy waternuclear reactor;

FIG. 3 is a schematic representation of another example of heavy waternuclear reactor;

FIG. 4 is a flow chart illustrating an example of a method of collecting³He gas;

FIG. 5 is a plot of moderator tritium activity in the moderator as afunction of time for one known reactor;

FIG. 6 is a plot of moderator tritium activity in the moderator as afunction of time for another known reactor; and

FIG. 7 is a plot of accumulated ³He and moderator tritium activity as afunction of time.

Elements shown in the figures have not necessarily been drawn to scale.Further, where considered appropriate, reference numerals may berepeated among the figures to indicate corresponding or analogouselements.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover processes or apparatuses that differ from those describedbelow. The claimed inventions are not limited to apparatuses orprocesses having all of the features of any one apparatus or processdescribed below or to features common to multiple or all of theapparatuses described below. It is possible that an apparatus or processdescribed below is not an embodiment of any claimed invention. Anyinvention disclosed in an apparatus or process described below that isnot claimed in this document may be the subject matter of anotherprotective instrument, for example, a continuing patent application, andthe applicants, inventors or owners do not intend to abandon, disclaimor dedicate to the public any such invention by its disclosure in thisdocument.

Helium⁻³ (³He) is an isotope of helium, with ⁴He being the most commonisotope of helium by a large factor. ³He has applications in a varietyof industries including, for example, the nuclear safeguard, security,medical and oil and gas industries.

For example, ³He can be used in neutron detector apparatuses that can beused to detect nuclear and radiological materials. Such neutron detectorapparatuses may be used at border crossings, ports, airports and otherpoints of entry into a country in an attempt to help detect smuggledand/or concealed nuclear material.

In other examples, ³He may be used in combination with magneticresonance imaging (MRI) to help provide visualization of a patient'slung capacity and function and/or may be used to help determine the rockporosity and/or presence of hydrocarbon reserves in the oil and gasindustry. In the construction industry, neutron detectors utilizing ³Hemay be used to measure soil compaction and moisture content. ³He mayalso be used to obtain low refrigeration temperatures via dilutionrefrigeration.

³He gas can be produced by the decay of the radioactive isotope tritium(³H), which has a half life of 12.3 years.

One source of ³He is tritium found in thermonuclear warheads. As thetritium decays it produces ³He. Tritium has also been produced throughneutron irradiation of ⁶Li-containingtritium-producing-burnable-absorber rods (TPBARs) in light water nuclearreactors. However, quantities of tritium produced in this manner, andthe resulting quantities of ³He produced by the decay of the tritium,may not be sufficient to satisfy ³He demand.

As the demand for neutron detectors and other commercial uses of ³He gasincreases the demand for ³He will also increase. Conventional sources of³He, such as harvesting ³He from decaying tritium in nuclear warheads,may not be sufficient to meet increased ³He demands. Some currentestimates suggest that the annual global demand for ³He gas now exceedsthe current annual supply of ³He gas. For example, while there is arelatively small amount of data regarding the current use of and/ordemand for ³He, is the inventors estimate that the production of ³He gasin the United States may be approximately 8,000 L/year, while the globaldemand for ³He gas is estimated to be about 65,000 L/year, or 65m³/year.

Accordingly, the inventors have identified a need for an alternativemethod of harvesting or collecting ³He gas. The inventors havediscovered that heavy water nuclear reactors may be one viable sourcefrom which ³He gas may be directly extracted or collected. It has beendiscovered that potentially useable amounts of ³He are produced withinthe heavy water contained the reactors, as either a moderator, coolantor both, and that this ³He can be directly harvested or extracted fromthe reactors without first separating, collecting and/or storing tritiumoutside the reactor. This direct extraction of ³He may be used as analternative to, or in combination with known tritium collectionprocesses. Some examples of heavy water reactors include pressurizedheavy water reactors (such as Canada Deuterium Uranium (CANDU™)reactors), reactors including a heavy water moderator, reactors that useheavy water as a coolant and reactors that use heavy water as both acoolant and a moderator. Whether utilized as a moderator and/or acoolant, heavy water that is present within the nuclear reactor may besubject to thermal neutron activation to produce tritium, and decay ofsuch tritium may form ³He.

Some commercial heavy water power reactors, such as CANDU™ reactors useheavy water (D₂O) in the moderator and heat transport systems. In suchreactors, the moderator may be contained within a calandria vessel, anda cover gas, such as a moderator cover gas, is provided within thecalandria in fluid communication to with free surface(s) of themoderator. Heavy water free surfaces may be present within the calandriavessel, and may also be present at one or more locations in otherprocess piping, vessels and other portions of a moderator system.

The space above some or all of these free surfaces is filled with themoderator cover gas via a cover gas system. The free surfaces of themoderator heavy water do not need to be in communication with eachother, and the moderator system may include multiple discrete regions inwhich moderator cover gas is in contact with a free surface of the heavywater. Optionally, all of the moderator cover gas can be circulatedwithin a common cover gas system.

Typically, the moderator cover gas is substantially pure helium gas(⁴He). For example, the cover gas may be at least 85% ⁴He and be atleast 90%, at least 95% and/or at least 99% ⁴He by volume when thereactor is in use. When the reactor is in operation, one or more othergases and/or impurities may accumulate within the cover gas. Forexample, D₂ may be produced from radiolysis of the heavy water and maycollect in the cover gas. Similarly, small amounts of O₂ (for examplefrom radiolysis of heavy water, from O₂ added to promote D₂—O₂recombination and/or from air leaks), N₂ (for example from air leaks),CO₂ possibly containing trace ¹⁴C (for example produced as an activationproduct of ¹⁷O in the moderator), ⁴¹Ar (an activation product of ⁴⁰Arwhich may be an impurity in the helium cover gas) and other gases, suchas T₂ and DT may also accumulate in the cover gas.

A moderator cover gas system is provided to circulate the cover gaswithin the reactor and within each gas head space, and may include anysuitable components and/or apparatus to help control the concentrationof impurities in the cover gas or otherwise process the cover gas,including, for example, cover gas preheaters, recombination units,scrubbers, catalytic converters and flame arresters. Optionally, thecover gas can be processed to help control the concentration ofimpurities in the helium cover gas within desirable design limits (forexample, less than about 3% H₂ by volume, less than about 2% O₂ byvolume, and less than about 4% D₂ by volume). The cover gas system mayalso include one or more sources of fresh, pure helium, including, forexample, helium bottle stations.

CANDU™ and other heavy water reactors may generate tritium (³H) in theheavy water systems as a waste by-product during operation (for example,when used to generate electrical power). For example, tritium (³H) maybe produced within the moderator through thermal neutron activation ofdeuterium (²H), via ²H(n,γ)³H, and in the heat transport system (orcoolant) of the heavy water reactor. The neutron radiative capturereaction ²H(n,γ)³H is believed to be the dominant method of tritiumproduction in heavy water reactors.

Some existing heavy water reactor facilities are configured to extracttritium from the heavy water used in the moderator and heat transportsystems, for example using heavy water detritiation plants, to helpreduce operator dose and environmental emissions. In such installations,the elemental tritium removed from the pressurized heavy water heattransport systems can be stored as titanium tritide in stainless steelstorage vessels as a waste product. Eventually, the tritium will decayproducing ³He gas. Such storage vessels may or may not includemechanisms for off-line recovery of the ³He gas. As these knownprocesses include separating and storing tritium from the reactors andthen harvesting ³He from the stored tritium, they may be referred to asin-direct ³He extraction processes.

While some ³He gas may be produced by the decay of tritium extractedfrom waste water storage tanks, the inventors believe that when heavywater reactors are in use a potentially useable quantity of ³He gas canbe produced within the heavy water moderator and/or the heavy watercoolant, and that at least some of the ³He gas can be directly extractedfrom these systems within the heavy water reactor (i.e. without firstharvesting and/or storing tritium)

For example, the inventors believe that ³He may be produced within themoderator and that at least some of the ³He present in the moderator canescape the liquid (e.g. via diffusion and/or via bubbling up to the freesurfaces surface) and may collect in a moderator cover gas that isprovided over the free surfaces of the moderator. The heavy watermoderator may be contained at relatively low pressures (relative to theheavy water used in the heat transport system), and may be atapproximately atmospheric pressure.

Alternatively, or in addition, ³He may be produced within the heavywater coolant, and may escape the coolant and be collected in a covergas provided in the heat transport system, including, for example thepressurized cover gas contained in the coolant pressurizer.

Instead of, or in addition to, removing and treating tritium-carryingheavy water from the heat transport system of the reactor, the inventorshave discovered that at least a portion of the moderator cover gas, andoptionally the coolant cover gas, can be extracted from reactor and canbe treated or processed to separate the ³He gas from the cover gas.Extracting and processing the moderator cover gas and/or coolant covergas may help collect at least some of the ³He gas produced in themoderator liquid. Conventional methods of collecting and storingtritium-carrying heavy water from the moderator or heat transportsystems do not capture the ³He gas that is directly released from themoderator liquid and coolant and accumulates in the cover gases. In someconfigurations, extracting ³He from the moderator cover gas system maybe more desirable than extracting ³He from the coolant cover gas becausethe heat transport system is an important safety system and it may notbe desirable to modify or interfere with such a system.

The separated cover gas and/or extracted ³He gas can then be storedand/or sent for further processing. Separating ³He gas from the covergas may, in some instances, be more desirable than processing waste heattransport heavy water and/or may help facilitate capture and collectionof the ³He gas that is generated within the reactor and and escapes fromthe heavy water prior to the collection, processing and/or storage ofthe heavy water.

For example, in heavy water reactors, ³He gas may be formed in themoderator as a result of tritium β⁻ decay. The ³He gas formed may alsobe converted back to tritium via the reaction ³He(n,p)³H, in themoderator. The thermal neutron absorption cross-section of the³He(n,p)³H reaction, in which the product of the tritium β⁻ decay isconverted back to tritium, is believe to be about seven orders ofmagnitude greater than the cross-section for the reaction ²H(n,γ)³H.However, it is believed that because of the low solubility of ³He gas inthe heavy water moderator, at least a portion of the ³He gas formed inthe moderator may escape irretrievably into the moderator cover gasprovided above the moderator, before this back conversion reaction canoccur. The ³He gas then mixes with the ⁴He forming the cover gas.Because it has been found that at least some ³He gas escapes themoderator, it is believed that the residence time of the ³He gas in themoderator may be too short to convert a significant amount of the ³Hegas formed in the moderator back into tritium. Therefore, it is believedthat there is the potential to recover useable and possibly commerciallysignificant quantities of ³He gas formed in the moderator of CANDUreactors by extracting and processing the moderator cover gas. Anexample of an estimate of ³He gas production in a typically CANDUreactor is set out below.

If one assumes that the extent of the ³He(n,p)³H reaction is negligiblecompared to the rate of ³He formation by tritium β⁻ decay, then the rateof ³He production in the moderator is given by the rate of tritium β⁻decay. Consequently, the ³He production rate (atoms·s⁻¹) can be writtenas:

$\begin{matrix}{\frac{d\; N_{3_{He}}}{d\; t} = {\lambda\;{MN}_{T}}} & (1)\end{matrix}$Where:M=Total heavy water inventory in the Moderator (kg),N_(3He)=Total number of ³He atoms in the moderator at time t (atoms),N_(T)=Number of tritium atoms in the moderator per kg_(D2O) at time t(atoms·kg⁻¹),t=Time (s), andλ=Tritium decay constant (s⁻¹).

To solve Equation 1, an estimate of the tritium concentration in themoderator as a function of time, t, is required. This can be done byusing an example a mass balance for tritium in the moderator:

$\begin{matrix}{{\frac{d}{d\; t}\left( {MN}_{T} \right)} = {{{\varphi\sigma}\; N_{D}m\;\alpha} + {F_{M}N_{0}} - {\lambda\;{MN}_{T}} - {L_{R}N_{T}}}} & (2) \\{{\frac{d\; N_{T}}{d\; t} + {N_{T}\left( {\lambda + \frac{L_{R}}{M}} \right)}} = {{{\varphi\sigma}\; N_{D}\frac{m}{M}\alpha} + {\frac{F_{M}}{M}N_{0}}}} & (3)\end{matrix}$Where:F_(M)=Make-up heavy water flow (kg·s⁻¹),L_(R)=Heavy water loss rate (kg·s⁻¹),N_(D)=Number of deuterium atoms in the moderator per kg_(D2O)(atoms·kg⁻¹),N₀=Number of tritium atoms in the make-up heavy water per kg_(D2O)(atoms·kg⁻¹),a=Reactor capacity factor,m=Heavy water inventory under the neutron flux (kg),t=time (s),φp=Thermal neutron flux (neutrons·cm⁻²·s⁻¹), andσ=Thermal neutron absorption cross-section (cm²)

Equation 3 is derived based on the simplifying assumption that theconversion of ³He, the tritium β⁻ decay product, back to tritium in themoderator is negligible. The general solution to Equation 3 is given by:

$\begin{matrix}{{N_{T} = {{{N(0)}e^{{- \lambda_{x}}t}} + {\frac{\left( {S + {\frac{F_{M}}{M}N_{D}}} \right)}{\lambda_{e}}\left\lbrack {1 - e^{{- \lambda_{e}}t}} \right\rbrack}}}{{Where}\text{:}}{S = {{\varphi\sigma}\; N_{D}\frac{m}{M}\alpha}}{{\lambda_{e} = {\lambda + \frac{L_{H}}{M}}},{and}}{{N(0)} = {{Initial}\mspace{14mu}{tritium}\mspace{14mu}{activity}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{{moderator}.}}}} & (4)\end{matrix}$

The heavy water loss rate may vary from reactor to reactor and there isno single value that can be used to describe a standard CANDU 6 reactor.For this reason, the case where, N₀=0 and L_(R)=F_(M)=0, was used tosimplify Equation 4 as given by (it is believed that the simplifyingassumptions used here lead to an overestimation of the tritium activityin the moderator):

$\begin{matrix}{N_{T} = {{{N(0)}e^{{- \lambda}\; t}} + {\frac{S}{\lambda}\left( {1 - e^{{- \lambda}\; t}} \right)}}} & (5)\end{matrix}$

Equation 5 based on the simplifying assumptions, described above, mayoverestimate the tritium activity in the moderator. The specific tritiumactivity (Bq·kg⁻¹) in the heavy water at time t can now be written as:A=λN _(T) =λN(0)e ^(−λt) +S(1−e ^(λt))  (6)For N(0)=0,A=S(1−e ^(−λt))  (7)

The tritium activity (A) in the moderator (Bq·kg⁻¹) of a CANDU reactorcan be estimated from Equation 6 provided that all the parameters in theequation are known. These values were used to calculate the tritiumactivity in the moderator of a CANDU 6 reactor as a function of time.The parameter values are shown in Table 1 [adopted from M. J. Song, S.H. Song, C. H. Jang, Waste Management, 15, 8, 593 (1995)].

TABLE 1 Parameter Values in Equation 3 for a CANDU 6 Reactor ParameterValue F_(M) Make-up heavy water flow (kg · s⁻¹) Reactor Dependant L_(R)Heavy water loss rate (kg · s⁻¹) Reactor Dependant M Total heavy waterinventory in the Moderator (kg) 2.57 × 10⁵ N_(T) Number of tritium atomsin the moderator per kg Variable D2O (atoms · kg⁻¹) N_(D) Number ofdeuterium atoms in the moderator per kg 6.01 × 10²⁵ D2O (atoms · kg⁻¹)(The isotpic purity of heavy water in the moderator ≥99.75%) N₀ Numberof tritium atoms in the make-up heavy water 0 per kg_(D2O) (atoms ·kg⁻¹) a Reactor capacity factor (A value of 85% is assumed) 85% m Heavywater inventory under the neutron flux (kg) 1.90 × 10⁵ ϕ Thermal neutronflux (neutrons · cm⁻² · s⁻¹) 2.30 × 10¹⁴ σ Neutron absorptioncross-section (cm²) 4.19 × 10⁻²⁸ λ Tritium decay constant (s⁻¹) 1.78 ×10⁻⁹

The use of Equation 6 to estimate the evolution of the moderator tritiumactivity with time was validated by comparing the data calculated usingEquation 6 and actual moderator activity data from two existing CANDUreactors. These comparisons are shown in FIG. 5 and FIG. 6,respectively. Considering the simplifying assumptions used in thederivation of Equation 6, the calculated moderator activity valuesfollow the measured values reasonably well. As expected the calculatedvalues were higher than the measured values due to a variety ofassumptions used in the derivation of Equation 6, including for exampleno loss of tritium from the moderator, other than from decay, a capacityfactor of 85% and flux of 2.30×10¹⁴. Also these results suggest that thecontribution of reaction ³He(n,p)³H to the production of ³H in themoderator is not important. The results confirm the validity of Equation6 and hence Equation 5 for use in estimating the tritium activity in themoderator in CANDU reactors, under the assumption that there is no lossof tritium from the moderator other than from decay.

The ³He production rate in the moderator (Equation 1) can now be writtenas:

$\begin{matrix}{\frac{d\; N_{3_{He}}}{d\; t} = {{\lambda\;{MN}_{T}} = {{\lambda\;{{MN}(0)}e^{{- \lambda}\; t}} + {{SM}\left( {1 - e^{{- \lambda}\; t}} \right)}}}} & (8) \\{{N_{3_{He}} = {{SMT} + {\left( {{{MN}(0)} - \frac{SM}{\lambda}} \right)\left( {1 - e^{{- \lambda}\; t}} \right)}}}{{{For}\mspace{14mu}{the}\mspace{14mu}{case}\mspace{14mu}{where}\mspace{14mu}{N(0)}} = {0.\mspace{14mu}{Equation}\mspace{14mu} 9\mspace{14mu}{simplifies}\mspace{14mu}{to}\text{:}}}} & (9) \\{N_{3_{He}} = {{SMt} - {\left( \frac{SM}{\lambda} \right)\left( {1 - e^{{- \lambda}\; t}} \right)}}} & (10)\end{matrix}$

Equation 8 gives an upper-bound estimate of the production rate of ³Hein the moderator for a CANDU 6 reactor and Equation 9 gives anupper-bound estimate of the total number of ³He atoms in the moderatoras a function of time. FIG. 7 shows the upper-bound estimates for total³He produced in the moderator and moderator tritium activity as afunction of time.

An estimate of the design life of current CANDU 6 reactors and thepressure tubes, at a capacity factor of 85%, may be 40 and 25 years,respectively. As FIG. 7 shows, over the design life of the pressuretubes, a typical CANDU 6 reactor generates about 12.7 m³ (STP) of ³He inthe moderator, assuming that there is no loss of tritium from themoderator through heavy water leaks, replacement, detritiation, orevaporation to the moderator cover gas. This amounts to an upper-bound,average ³He production rate of ˜0.8 m³ (STP) per year. As the data showan amount of ³He (<<1 m³ (STP) per annum) may be available for recoveryfrom a CANDU reactor from the moderator cover gas.

The tritium activities in the moderators and the ³He production rates atdifferent CANDU reactors in Canada are different. Table 2 shows the ³Heproduction rate as a function of the tritium activity in the moderatorfor a typical CANDU 6 reactor. As the data show, even at the highestmoderator activity used in the calculations, which is similar to themeasured tritium activity in the Existing Reactor Two moderator in 2007,the ³He production rate is <0.7 m³(STP)·per year.

TABLE 2 Estimated ³He Production Rate per Year in a CANDU 6 ReactorModerator Activity (GBq · kg⁻¹) ³He Production Rate (m³(STP) · a⁻¹) 3700.11 740 0.22 1110 0.34 1480 0.45 1850 0.56 2220 0.67

Based on the above, it is believe that the amount of ³He gas in theextracted moderator cover gas is significantly higher than the amount ofnaturally abundant ³He found in the helium cover gas. That is, themoderator cover gas within the calandria may be ³He enriched. If ³He gasis to be extracted from collected moderator cover gas, it may bedesirable to capture a significant portion of the ³He enriched cover gasin the calandria, and preferably to capture substantially all of the ³Heenriched cover gas, for processing.

In operation, some of the moderator cover gas may escape from thecalandria and/or the moderator cover gas system. If, for example, adaily helium loss rate of 30% of the total helium inventory in themoderator cover gas system is assumed, it is believed that recoveringabout 800-900 m³ per year of helium may help facilitate recovery of mostof the ³He produced in a CANDU 6 reactor. Table 3 shows theconcentration of ³He in the recovered helium gas, as a function of theaverage moderator activity.

TABLE 3 Estimated Concentraion of ³He in the Recovered Helium GasModerator Activity (GBq · kg⁻¹) ³He in Recovered Helium (ppm V) 370 130740 260 1110 410 1480 540 1850 670 2220 800

Preferably, the cover gas system can be configured so that impurities inthe moderator cover gas, including, for example, D₂, O₂, CO₂, ¹⁴C, ⁴¹Ar,T₂ and DT, can be removed from the helium cover gas before the cover gasis processed to separate the ³He gas from the ⁴He gas.

Optionally, the extraction of the cover gas can be an off-line process,when the reactor is shut down and/or the cover gas system(s) are purgedallowing substantially all of the enriched cover gas to be collected ina single batch. This may allow the cover gas to be batch processed toextract the ³He gas, which may be advantageous for some extractionprocess and/or apparatuses. It may also be desirable if the reactor isgoing to be shut down anyway (for example for service).

Alternatively, the extraction of the cover gas can be an on-lineprocess, in which a stream (optionally a generally continuous stream) ofcover gas can be drawn from the calandria and/or the heat transportsystem (e.g. the pressurizer) while the reactor is in use. Optionally,in such a configuration the ³He gas can be separated from the extractedstream of cover gas using a real time or on-line process or separationapparatus. This may allow the ³He gas to be extracted while the reactoris in use. This may help facilitate substantially continuous collectionof ³He gas

Optionally, after the ³He gas has been separated, some or all of thecover gas extracted from the calandria can be recycled and reintroducedinto the calandria. Cover gas from the heat transport system may also betreated and recycled. Recycling at least some of the cover gas may helpreduce the amount of make up or replacement cover gas needed and/or mayhelp increase the efficiency of the cover gas system.

The ³He gas can be separated from the ⁴He cover gas using any suitableseparation apparatus and/or separation technique. For example, a numberof technologies have been used in the past for separating ³He from³He+⁴He mixtures. Examples of some of the known methods include:

1. Thermal Diffusion

2. Fractional Distillation

3. “Heat-Flush” Method

4. “Super Leak” Method

5. Cryogenic Adsorption

Thermal Diffusion

Before the extraction of ³He from tritium decay started, there have beenefforts to separate naturally abundant ³He from helium sources(extracted from air or natural gas). Thermal diffusion has one of theearly technologies tested for use in enriching naturally abundant ³He inhelium sources.

Thermal diffusion is the relative motion of the components of a gaseousmixture or solution, which is established when there is a temperaturegradient in a medium. Thermal diffusion in gases was theoreticallypredicted by Enskog on the basis of the kinetic theory of gases [D.Enskog, Physik Zeits, 12, 56 and 533 (1911)]. It was later discoveredexperimentally by Chapman and Dootson [S. Chapman, F. W. Dootson, Phil.Mag., 33, 248 (1917)]. Thermal diffusion sets up a concentrationgradient with lighter molecules concentrating at the high-temperatureside with heavier molecules concentrating in the low-temperature sideleading to separation of components in a gaseous mixture. Theconcentration gradient in turn causes ordinary diffusion and theseparation effect of thermal diffusion is balanced by the counteractionof the concentration diffusion.

In a binary gaseous mixture at constant pressure, the total diffusionmass flux, J_(i), for each component, i, in the absence of externalforces, is given by:

$\begin{matrix}{J_{i} = {{{{- {nD}_{12}}{\nabla\; C_{i}}} + {{nD}_{T}\frac{1}{T}{\nabla\; T}}} = {{- {{nD}_{12}\left\lbrack {{\nabla\; C_{i}} - {k_{T}\frac{1}{T}{\nabla\; T}}} \right\rbrack}} = {- {{nD}_{12}\left\lbrack {{\nabla\; C_{i}} - {{aC}_{1}C_{2}{\nabla\;\ln}\; T}} \right\rbrack}}}}} & (11)\end{matrix}$Where:

C_(i)=Concentration of species i (C_(i)=n_(i)/n, 1=1,2)

D₁₂=Binary diffusion coefficient,

D_(T)=Thermal diffusion coefficient,

n=Total number of molecules in unit volume (n=n₁+n₂)

k_(T)=Thermal diffusion ratio=D₁₂/D_(T)=αC₁C₂

α=Thermal diffusion constant

In gaseous mixtures α does not generally exceed 0.4; and for mixture ofisotopes, a typical value for α is ˜0.01. The value of k_(T) depends ina complex manner on the molecular masses, effective molecule size,temperature, mixture composition, and on the laws of intermolecularinteraction. The closer the intermolecular forces approach the laws ofinteraction between the elastic, solid, spheres, the greater is thevalue of k_(T); it also increases with increase in the moleculedimension and mass ratio. When molecules interact in accordance with thelaw for solid, elastic, spheres, k_(T) is independent of the temperatureand the heavier molecules gather, in this case, in the cold region(k_(T)>0 for m₁>m₂>2 where m₁ and m₂ are the masses of the respectivecomponents), but if m₁ and m₂ are equal, then larger molecules move intoa cold region. For other laws of intermolecular interaction k_(T) candepend considerably on the temperature and can even change sign.

Thermal diffusion became important as a method of separating isotopes ormixtures of gases when Clusius and Dickel invented the thermal-diffusioncolumn [K. Klusius, G. Dickel, Naturwiss, 26, 546 (1938)]. The originalthermal diffusion column theory was developed by Furry, Jones andOnsagar (FJO) [W. H. Furry, R. C. Jones, L. Onsager, Phys. Rev., 55,1083 (1939)], [W. H. Furry, R. C. Jones, Phys. Rev., 69, 459 (1946)],[R. C. Jones, W. H. Furry, Rev. Mod. Phys., 18, 151 (1946)]. A thermaldiffusion column, used for isotopes separation, essentially consists ofa vertical tube maintained at a low temperature with a heated wirelocated in the central axis. Other variants include coaxial,tube-in-tube configuration, where the central tube is heated andmaintained at a high temperature while outer tube is maintained at alower temperature. In a thermal diffusion column, the lighter gas flowsupwards near the central hot wire or tube and the heavier gas flowsdownwards near the outside cold wall, by convection. The temperaturegradient across the tube causes a horizontal concentration gradient bythermal diffusion with the lighter molecules concentrating at the hotcentral wire or tube and the heavier molecules concentrating at the coldwall. These two effects are superimposed and the opposing convectioncurrents carry the lighter molecule to the top and the heavier moleculeto the bottom. The upward and downward gas flows are in counter flowresulting in a concentration gradient between the top and the bottom ofthe column greater than in a horizontal plane. The maximum separationfactor that can be obtained in a column is limited by the remixing ofgases caused by ordinary or concentration diffusion and by theconvection currents. For a binary mixture, modified set of FJO equationsdescribing the mass transport of species in the Thermal Diffusion columnis given in [L. Hodor, Sep. Sci. Technol., 38, 5, 1229 (2003)].

The effectiveness of thermal diffusion as a means of separating ³He fromoil-well helium (³He/⁴He=1×10⁻⁷) has been investigated by severalgroups. McInteer et al. [B. B. McInteer, L. T. Aldrich A. O. Nier, Phys.Rev. 74, 8, 946 (1948)], using a 3-column thermal diffusion system, wereable to produce 14 cm3(STP) per day of 0.21% ³He using a 1.15×10⁻⁵% ³Hefeed, which corresponds to a separation factor of 1.83×10⁴, under thetest conditions. The thermal diffusion system used consisted of two 3.5m-long coaxial, tube-in-tube columns in the front end. The first columnhad a hot wall diameter of ˜6.04 cm and cold wall diameter of 7.3 cm andthe second column had a hot wall diameter of 3.5 cm and a cold walldiameter of 4.76 cm. The final column consisted of a 2.5 m-long hot-wirecolumn of wire diameter 0.036 cm. The columns were operated at highpressure (0.69 and 0.88 MPa(g)). The separation factor achieved, underthe tested conditions, was found to be a strong function of the productdraw-off rate and it decreased with increasing draw-off. It was alsofound that the hot-wire column alone could have a separation factor inthe order of 1×10⁴.

A thermal diffusion plant capable of producing 2 cm³ (STP) per week wasalso operated for several years at an existing establishment at Harwell,England for several years using a feed gas containing naturally abundant³He in helium from air (1.2×10⁻⁴% ³He). The thermal diffusion systemconsisted of two identical coaxial, tube-in-tube columns (0.8 cmdiameter hot wall×3 cm diameter cold wall×4.5 m high) at the front endand a hot-wire column (1.3 cm diameter cold wall×4.57 m high) at theback end.

With the feed gas that is available from CANDU reactors, it is believedthat an overall separation factor of about 1×10³ to about 1×10⁴ may berequired to obtain a stream of ³He with 99.9% purity. However, for therecovery of ³He from the available feeds, a large volume of recoveredhelium gas (˜840 m³) needs to be processed. While thermal diffusion is arelatively straightforward method of isotope separation, as a separationprocess, it may have a low thermodynamic efficiency, requires multiplestages, large amounts of electrical power and long processing times.However, thermal diffusion may be suitable as a final stage of ³Heenrichment since, a single hot-wire column could have a separationfactor in the order of 1×10⁴.

Fractional Distillation

The boiling points of ⁴He and ³He are 4.2 K and 3.9 K respectively and³He+⁴He mixtures may be separated by distillation. Distillation of³He+⁴He solutions is generally considered as a more efficient methodthan the thermal diffusion method for the separation of ³He isotope. Theseparation factor for thermal diffusion is proportional to the squareroot of the isotopic mass ratio which is fixed at 1.5 while theseparation factor for distillation is proportional to the relativevolatility ratio and the minimum separation factor for ³He+⁴Hedistillation is 2.5 at the critical temperature of 3.36 K, and increaseswith decreasing temperature. Several small-scale, batch distillationprocesses have been reported for the purification of ³He+⁴He mixturesrelatively rich in ³He [W. R. Abel, A. C. Anderson, W. C. Black, J. C.Wheatley, Physics, 1, 337 (1967)], [V. N. Grigor'ev, B. N. Yesel'son, V.A. Mikheev, O. A. Tolkacheva, Sov. Phys., J.E.T.P., 25, 572 (1976)], [R.H. Sherman, Proceedings of the 10th International Conference onLow-Temperature Physics, Vol. 1, 188 (1966)], [R. P. Giffard, R. B.Harrison, J. Hatton, W. S. Truscot, Cryogenics, 7, 179 (1967)], [A. C.Anderson, Cryogenics, 8, 50 (1968)], [A. Tominaga, S. Kawano, Y.Narahara, J. Phys., D: Appl. Phys. 22, 1020 (1989)]. The ³He in the feedused in these studies varied from 10 to 99.9993% and ³He product purityvaried from 99.99% to 99.9998%. The production rate of ³He in theseprocesses was reported to be in the range 0.1 to 18.5 L·h⁻¹. Theobjective of most of these processes was essentially to remove the ⁴Heimpurity traces in enriched ³He. These data, however, show the potentialof the fractional distillation process to obtain very high purity ³He.

A continuous distillation apparatus for the separation of ³He-⁴Hemixtures is described in W. R. Wilkes, Advances in CryogenicEngineering, 16, 298 (1970), the entirety of which is incorporatedherein by reference. This system has been operated continuously for fewhours at a time with a ³He-⁴He mixture containing 8.7% ³He in the feedwhile withdrawing a product containing 99.95% ³He and a raffinatecontaining 0.02% ³He. Based on the data obtained, the author concludedthat at a feed rate ˜60 L(STP)·h⁻¹ of this mixture, a 99.9% pure ³Heproduct may be obtained at a rate of ˜5.2 L·h⁻¹. A continuousdistillation process of this size is suitable as a final stage ofenriching ³He in ³He+⁴He mixtures recovered from the moderator covergas. However, this requires pre-enriching the ³He content in ³He+⁴Hemixtures, recovered from the moderator cover gas from CANDU reactors,for use as feed for a distillation system of similar size.

Heat Flush Method

The “heat flush” method of ³He-⁴He isotopes separation is based onsuperfluid properties of ⁴He. The “heat flush” method exploits theproperty that ³He does not participate in the superfluid flow of ⁴He. Ithas been demonstrated that if heat is applied at one end of a vesselcontaining liquid helium below the lambda point (λ) and refrigeration atthe other end, the ³He flows with the normal liquid away from the heaterand towards the cold end of the vessel [C. T. Lane, H. A. Fairbank, L.T. Aldrich, A. O. Nier, Phys. Rev. 73, 256 (1948)], [T. Soller, W. M.Fairbank, A. D. Crowell, Phys. Rev. 91, 1058 (1953)]. The λ point is thetemperature below which normal fluid helium (helium I) transitions tothe superfluid helium point (helium II).

The A temperature of ⁴He in ³He+⁴He solutions decreases with increasing³He content in the solution. Consequently, at any given temperaturethere is a ³He concentration, above which ⁴He is no longer a superfluid.This, in effect, imposes a limitation on the enrichment of ³He that canbe achieved using methods that exploit superfluid properties of ⁴He. The“heat flush” method has been used to enrich gas-well helium (˜1×10⁻⁷) bya factor of 130 [C. T. Lane, H. A. Fairbank, L. T. Aldrich, A. O. Nier,Phys. Rev. 73, 256 (1948)] and 3×10⁴. In the latter case, up to 0.5% of³He in ³He+⁴He mixtures were obtained at a rate of about 60-75 cm³ ofenriched gas. A device combining the “heat flush” method with batchdistillation, capable of enriching ³He from natural abundance level(˜1×10⁻⁸) to ˜99.5% has also been demonstrated [V. P. Peshkov, J. Exp.Theor. Phys, 30, 850 (1956), Translation: Soviet Physics, JETP, 3, 706(1956)]. In this case, product of the “heat flush” step was ˜0.2% of³He. There have been no reports of enrichment of ³He in ³He+⁴He mixturesabove ˜1.5% directly by the “heat flush” method. A final enrichment upto 4% of ³He has been achieved by the “heat flush” method usingpre-enriched mixture of ³He-⁴He up to 0.01% ³He by thermal diffusionmethod. While the “heat flush” method may not achieve enrichment of ³Hesignificantly above few percent, it may be used as a suitablepre-enrichment process for separating ³He from the moderator cover gasrecovered from CANDU reactors.

Superleak Method

The “superleak” method of separating ³He-⁴He gas mixtures is also basedon the superfluid properties of ⁴He. The method of “superleak” is basedon the ability of superfluid ⁴He to flow through capillaries or verynarrow channels, while ³He cannot. This method has been used topartially enrich ³He present in atmospheric helium at an abundance ratioof 1.22×10⁻⁶. The “superleak” consisted of a ground glass joint with achannel width of ˜1 μm. Using ten parallel superleaks of dimension1×10⁻⁴ cm, a ³He+⁴He mixture containing 2% ³He was enriched to 95%, in asingle operation, while processing the initial gas mixture at a rate of200 cm³ (STP) per hour. Since the “superleak” method is believed to becapable of enriching ³He to significant levels from relatively dilute³He+⁴He mixtures in a single operation, the “superleak” process may be asuitable pre-enrichment process that could be coupled to a fractionaldistillation stage to achieve high-purity ³He.

Cryogenic Adsorption Method

The adsorption based separation of ³He-⁴He, at liquid heliumtemperature, is based on the differences in the adsorption energies of³He and ⁴He on activated charcoal. This method has been used to removetrace amounts of ⁴He impurity (˜0.1%) in commercially available ³He. Areduction in the ⁴He impurity from 0.1% to <0.01% has been achievedafter two passes through the column of 25 L of at a flow rate in therange 0.04-0.1 L·min⁻¹. About 23 L of purified product and ˜2 L of ⁴Heenriched gas were recovered. No details on the amount of charcoal usedin the process or the ⁴He adsorption capacity of charcoal at liquidhelium temperature are given. No other reports on using the cryogenicadsorption of ⁴He on charcoal at high ⁴He levels are found in theliterature. This method, while simple, may be more appropriate forremoving ⁴He impurity at trace levels in enriched ³He.

Using one or more of the apparatuses and processes described above, andoptionally using any other suitable apparatus and/or process, there areseveral processing options are available for consideration for theprocessing of moderator cover gas recovered from CANDU reactors toextract high purity ³He. Some examples of processing options include:

-   1. Pre-enrichment with “heatflush” method and final enrichment with    distillation,-   2. Pre-enrichment with “superleak” method and final enrichment with    distillation, and-   3. Pre-enrichment with “superleak” method and final enrichment with    thermal diffusion.

Referring to FIG. 1, a schematic representation example of a heavy waterreactor, e.g. a CANDU reactor 100, includes a calandria 102 containing aheavy water moderator liquid 104 and a plurality of pressure tubes 106extending through the calandria 102. A heat transport system 108 is usedto circulate a cooling fluid 110 through the pressure tubes, andincludes a pressurizer 109. A moderator a cover gas system 112 is usedto circulate and optionally treat or process a moderator cover gas 114,and a coolant cover gas system 113 is used to circulate and optionallytreat a coolant cover gas 137. Optionally, the moderator cover gas 114and the coolant cover gas 137 may be the same gas, such as helium.

The pressure tubes 106 may be of any suitable design and can contain oneor more nuclear fuel bundles/rods 115. The reactor 100 can include anysuitable number of pressure tubes 106, arranged in any suitableconfiguration. The pressure tubes 106 can be formed from any suitablematerial.

The heat transport system 108 may be used to circulate a pressurizedheavy water cooling fluid 110 through the pressure tubes 106. Incomingheavy water cooling fluid enters the tubes 106, illustrated by arrows116, is heated by the fuel bundles 115 and exits the pressure tubes 106,illustrated by arrows 118 at an elevated temperature. The hightemperature cooling fluid may then flow through any suitable heatexchanger, for example a boiler 120 that can be used to heat an incomingwater stream 122 to generate a steam stream 124, which may in turn beused to drive any suitable turbine generator (not shown) and produceelectrical power. The heat transport system 108 may include any suitablefixtures and components including, for example, valves, pumps, filtersand any other suitable apparatus that is not illustrated in the presentschematic drawing.

The moderator liquid 104 is contained within the calandria 102 andsurrounds the pressure tubes 106. A moderator system 119 circulates themoderator through the calandria, and can include any suitable piping,conduits, processing modules (such as a heat exchanger), valves, pumpsand other such components. In the illustrated schematic, a moderatorvessel 117 holds moderator liquid that is outside the calandria.

The moderator liquid 104 may have exposed free surfaces 126 at aplurality of locations within the moderator system 115. For example, afree surface 126 is located toward the top of the calandria 102. A headspace or plenum 128 is defined between the free surface 126 of themoderator fluid 104 and the upper wall 130 of the calandria. Whileillustrated as a single, continuous chamber, the head space 128 may beformed from two or more separate chambers or regions within thecalandria 102, and need not be a single, continuous chamber. The sizeand shape of the head space 128 may be selected based on a variety offactors, including, for example the calandria size, the calandria shape,the configuration of the cover gas system 112 and the operatingconditions of the reactor 100.

A free surface 126 a may also be formed within a head space 128 a invessel 117, and optionally within some of the conduits or piping of themoderator system 115. Each head space 128 and 128 a may be filled withmoderator cover gas, and may be in fluid communication with a commonmoderator cover gas system 112. Cover gas system features described inrelation to the calandria 104 and head space 128 may also be included invessel 117 and head space 128 a, and analogous elements may beidentified using analogous reference characters with an “a” suffix.

The calandria 104 may include a gas inlet 132 and a gas outlet 134 thatare in fluid communication with each other, for example via the covergas head space 128, and that can be connected to any suitable cover gassystem 112. While illustrated as a single port for clarity, the gasinlet 132 may include a plurality of discrete ports or openings in thecalandria sidewall and the supplying conduit may have a correspondingnumber of branches and outlets. Similarly, the gas outlet 134 mayinclude a plurality of separate ports or openings that are incommunication with the cover gas head space 128, and connected to acommon outlet passage.

The cover gas 114 can flow into the head space 128 via the gas inlet 132and can be extracted from the head space 128 via the gas outlet 134.Optionally, the gas inlet 132 and gas outlet 134 can include anysuitable valve(s) or flow control mechanism to selectably adjust and/orlimit the flow of cover gas 114 within the head space 128. The gas inlet132 and gas outlet 134 may also include any other suitable equipment,including, for example, a flow meter and sensors.

The cover gas system is used to supply cover gas 114 to the calandria104 and to circulate the cover gas 114 through the head space 128. Thecover gas system 112 can be of any suitable configuration, and mayinclude any suitable components or apparatuses. In the illustratedexample, the cover gas system 112 includes a cover gas supply passage138 for supplying cover gas 114 to the head space 128, and a gasextraction passage 140 for extracting gas from within the head space128. The passages 138, 140 may be formed from any suitable conduitmembers, including, for example pipes and ducts, and may be formed fromany material that is suitable for use with a pressurized heavy waterreactor.

In the illustrated example, the cover gas supply passage 138 has anupstream or inlet end 142 and downstream or outlet end 144 that isspaced apart from the inlet end 142. The outlet 144 of the supplypassage 138 is connectable to the gas inlet 132 on the calandria 104. Inthis configuration, cover gas 114 may be supplied into the head space128 via the supply passage 138, as illustrated using arrows 146. Whencontained within the head space 128, the cover gas 114 is in contactwith the free surface 126 of the moderator liquid 104.

The inlet 142 of the cover gas supply passage 138 can be connected toany suitable supply or source of cover gas 114. In the illustratedexample, the inlet 142 of the cover gas supply passage 138 is connectedto a separation apparatus 148, as explained in greater detail below.Alternatively, the inlet 132 may be connected to a helium bottle (notshown) or other cover gas supply source.

As explained in detail above, when the reactor 100 is operated aquantity of tritium may be produced within the moderator liquid. Thetritium may then decay to produce ³He gas 150 in the moderator liquid104. Due to the relatively low solubility of ³He gas in the heavy watermoderator 104, at least a portion of the ³He gas 150 produced may formbubbles, diffuse out of the free surfaces 126 or otherwise escape fromthe moderator liquid, as illustrated using arrows 152. ³He gas bubblingout of the moderator liquid 104 can flow into the head space 128, andmay become mixed with the cover gas 114 contained in the head space 128.

Optionally, as explained above, the cover gas 114 introduced into thehead space may be substantially pure helium (⁴He) gas. When the ³He gas150, and other impurities and by-products as explained above, flow intothe head space 128, the composition of the cover gas 114 may change fromsubstantially pure helium (⁴He) to a mixture of gases. The mixture ofgases may be extracted from the head space 128 via the gas outlet 134 asa gas outlet stream, represented by arrow 154. In this configuration thegas outlet stream 154 may include a mixture of the helium cover gas 114and at least a portion of the ³He gas 150.

In the illustrated example, an inlet end 156 of the gas extractionpassage 140 is coupled to the gas outlet 134 of the calandria 104 toextract the gas outlet stream 150 from the head space 128. The gasoutlet stream 154 can flow along the gas extraction passage 140, awayfrom the head space 128, for further treatment and/or processing.

One or more suitable gas treatment and/or processing apparatuses can beprovided in the gas extraction passage 140, downstream from the headspace 128. Some examples of suitable gas processing apparatuses areexplained above. The processing apparatuses can be selected to processthe gas outlet stream in a variety of different ways. In addition to, oras an alternative to known gas processing apparatuses, the cover gassystem 112 includes a ³He gas separation apparatus 148 that is operableto separate ³He gas 150 from the mixture of gases forming the gas outletstream 154. The ³He gas separation apparatus 148 may be of any suitableconfiguration and can include one or more gas separation modules. Someexamples of suitable ³He gas separation apparatuses are explained indetail above, and any one of these apparatuses can be used alone and/orin combination with any one or more of the other apparatuses describedherein or any other suitable apparatus. One or more suitable gasprocessing apparatus (for example apparatuses to remove otherimpurities, such as D₂, O₂, CO₂ and ⁴¹Ar from the cover gas gas) may beprovided upstream and/or downstream from the ³He separation apparatus.

In the illustrated example, the ³He gas separation apparatus 148includes a ³He gas outlet passage 160 to output a stream 162 of ³He gasseparated from the outlet gas stream 154. The ³He gas outlet passage 160can be connected to any suitable downstream apparatus including, forexample, a storage container and/or secondary processing apparatus (notshown).

Optionally, the ³He gas separation apparatus 148 may also include atleast one other outlet to output non-³He gas streams. In the illustratedexample, the ³He gas separation apparatus 148 includes a second outlet164 for outputting a treated cover gas stream 166. The treated cover gasstream 166 may include the helium cover gas from which the ³He gas wasseparated, and may include other trace gases and/or impurities. In theillustrated example, the non-³He gas outlet 164 is coupled to the inlet142 of the cover gas supply passage 138. In this configuration, treatedcover gas 166 (i.e. cover gas that has had the ³He gas removed) can bere-used and recycled into the head space 128. Alternatively, the non-³Hegas outlet 164 need not be coupled to the cover gas supply passage 138,and the non-³He gas exiting the separation apparatus 148 can becontained or disposed of in any suitable manner. Optionally, one or moreadditional gas treatment apparatuses can be provided upstream ordownstream from the separation apparatus 148.

Optionally, the cover gas system 112 can be operated as an on-linesystem, in which the gas outlet stream 154 can be drawn from the headspace 128 while the reactor 100 is in use. In this configuration, thegas outlet stream 154 may be extracted at a generally uniform flow ratewhile the reactor 100 is in use. Alternatively, the flow rate of the gasoutlet stream 154 may vary over time and/or in response to operatingconditions of the reactor. Operating the cover gas system 112 in anon-line configuration may allow the ³He gas to continuously extractedfrom the head space 128 while the reactor 100 is in use. This may allowcollection of ³He gas from active reactors and may help to minimizedisruptions or alterations to the operating conditions of the reactor.

Alternatively, the cover gas system 112 can be operated in an off-lineor batch-type system.

It is understood that only some aspects of the reactor 100 areillustrated in the present schematic. An operational reactor 100incorporating one or more of the aspects of the present teaching mayinclude any combination of suitable operating components, including, forexample, control rods, light water condensate pumps, secondary coolingloops, fuel loading machines, a reactor containment building, apressurizer, valves, pumps and any other suitable equipment.

While treatment of the moderator cover gas 114, via a moderator covergas system 112 is described in detail above, an analogous process may beused to extract ³He from the coolant cover gas 137. A coolant cover gassystem 1112 may include some or all of the elements of the cover gassystems described herein, and/or may include additional elements notdescribed above. The coolant cover gas system 1112 may be generallysimilar to the moderator cover gas system 112, and analogous elementsare illustrated using like reference numerals indexed by 1000. Thecoolant cover gas system 1112 may include any suitable ³He separationapparatus 1148 that may be in fluid communication with head space 1128within the pressurizer 109 (or at any other suitable location orlocations within the coolant system 113). A cover gas extraction passage1140 may transport the coolant cover gas 1128, including ³He mixedtherein, for processing, and treated cover gas may be returned to thehead space 1128 via the gas supply passage 1138.

Referring to FIG. 2, another schematic example of a reactor 200 includesa calandria 202 and pressure tubes 206. The reactor 200 may be generallysimilar to reactor 100, and like elements are illustrated using likereference characters indexed by 100.

The calandria 202 contains a heavy water moderator liquid 204 and acover gas head space 228 is provided above the free surface 226 of themoderator liquid 204. A cover gas supply passage 238 is connected to gasinlet 232 to introduce the cover gas 236 into the head space 228, and agas extraction passage 240 extends away from the gas outlet 234 toextract a gas outlet stream 254 from the head space 228.

A ³He separation apparatus 248 is provided in the gas extractionpassage, downstream from the gas outlet. In the illustrated example, the³He separation apparatus 248 is a two-stage apparatus that includes afirst separation module 270 and a second separation module 272 provideddownstream from the first module 270. The first and second separationmodules 270, 272 may be the same apparatus/process, or alternatively maybe different apparatuses/processes. An intermediate conduit 274 extendsfrom an outlet 276 on the first separation module 270 to an inlet 278 onthe second module 272. A ³He gas outlet 280 on the second module 272 canform the ³He gas outlet for outputting a ³He gas stream 262.

At least one, or both, of the first separation module 270 and secondseparation module 272 may also include one or more second outlet 264 foroutputting a treated cover gas stream 266, or other gas output stream.Optionally, in the illustrated example, as illustrated using dashedlines, at least a portion 266 a of the treated cover gas 266 stream maybe recycled via a recycle passage 282 and re-introduced in the cover gassupply passage 238, upstream from the cover gas inlet 232 of thecalandria 202.

Optionally, one or more suitable treatment apparatuses may be providedin the recycle passage. In the illustrated configuration, two gastreatment apparatuses 284 a and 284 b are provided in the recyclepassage 282. The gas treatment apparatuses 284 a and 284 b may bepreheaters, recombination units, filters, separators, flame arresters orany other suitable apparatus. Passing the treated cover gas stream 266 athrough one or more treatment apparatuses 284 a and 284 b may helpremove additional impurities from the cover gas and otherwise treat thecover gas so that it is suitable for re-introduction into the head space228. This may help make the treated cover gas 266 a more suitable forre-use.

While illustrated as being provided in the recycle passage 282, the gastreatment apparatuses 284 a and 284 b may be provided in the gasextraction passage 240, and optionally, may be upstream from the ³Heseparation apparatus 248.

Alternatively, or in addition to receiving recycled cover gas 266 a, thegas supply passage 238 may be connected to any suitable external covergas source (not shown). The cover gas 236 supplied to the head space 228may comprise recycled cover gas 266 a, fresh cover gas 246 a or anysuitable combination thereof.

Referring to FIG. 3, another schematic example of a reactor 300 includesa calandria 302, pressure tubes 306 and a cover gas supply passage 338.A heavy water moderator liquid 304 is contained in the calandria 302 anda cover gas head space 328 covers the free surface 326 of the moderator304. The reactor 300 may be generally similar to reactor 100, and likeelements are illustrated using like reference characters indexed by 200

In the illustrated example, a gas treatment apparatus 384 is providedthe extracted gas passage 340 upstream from the ³He separation apparatus348. The gas outlet stream 354 can be extracted from the head space 328and fed into the gas treatment apparatus 384. Impurities and other gasesremoved by the gas treatment apparatus 384 can be discharged via a firstoutlet 390 as an impurity gas stream 392.

After treatment, a partially treated gas stream 394, for examplecomprising primarily ³He and ⁴He gases, can exit the gas treatmentapparatus 384 via a second outlet 398 and can flow into the ³Heseparation apparatus 348. A stream of separated ³He gas 362 can exit the³He separation apparatus 348 via a first outlet 400, and a stream oftreated cover gas 366 can exit the ³He separation apparatus via a secondoutlet 402. In some configurations, based on the performance andcharacteristics of the gas treatment apparatus 384 and the ³Heseparation apparatus 348, the stream of cover gas 366 exiting the ³Heseparation apparatus 348 may be substantially pure ⁴He gas.

Positioning one or more gas treatment apparatuses 384 upstream from the³He separation apparatus 348 may facilitate removal of impurities andother gases from the gas outlet stream 354 before the gas outlet streamreaches the ³He separation apparatus 348. This may help prevent foulingor damage to the ³He separation apparatus 348. This may also helpimprove the efficiency of the ³He separation apparatus 348 and/or allowfor the use of a particular ³He separation apparatus (providing a givenseparation process) that may not be suitable for use on a gas streamthat includes impurities or a mixture of gases other than ³He and ⁴Hegases.

Optionally, a gas recycle passage 382 can be provided to recycle some orall of the cover gas 366 exiting the ³He separation apparatus 348 to thecover gas supply passage 338 for re-introduction into the head space328.

In all the configurations shown in FIGS. 1, 2 and 3, the devices andequipment for separating out ³He can be provided in a separate circuitfrom the main circuit, possibly in parallel with it, to maintain aseparate circulation of the helium to the cover gas space above themoderator, so that any failure of equipment in the separate circuitshould not compromise operation of the main circuit. As noted it may bepreferable, in the separate circuit to provide elements to remove orotherwise process other contaminant gases. For example some form ofigniter can be provided to ensure that any hydrogen or deuterium presentis burned to form water. After removal or processing of suchcontaminants, the helium isotopes can be separated.

Referring to FIG. 4, a method of collecting ³He from a pressurized heavywater nuclear reactor may begin a step 1000 with providing heavy wateras either a moderator, coolant or both within a heavy water reactor. Atstep 1002 a cover gas can be provided within the reactor and may coverat least a portion of the free surface of the heavy water. Optionally,the cover gas may be substantially pure ⁴He, for example comprising atleast 90% ⁴He by volume. If necessary, additional make-up cover gas maybe added to the reactor from time to time, as needed, at step 1002 a.

At step 1004 the heavy water nuclear reactor may be operated to produce³He gas, for example via decay of tritium in the heavy water. At least aportion of the ³He gas may escape from the heavy water and mix with thecover gas, at step 1006.

At step 1008 an outlet gas stream is extracted from within the reactor.The outlet gas stream may include a mixture of the cover gas, the ³Hegas and other trace gases and/or impurities.

At step 1010 the ³He gas is separated from the outlet gas stream using asuitable separation apparatus. Optionally, the method can include theoptional step 1010 a of collecting or routing the ³He gas for furtherprocessing, and the optional step 1010 b in which the collected ³He gascan be further treated or purified.

At step 1012 a ³He gas stream may be output from the separationapparatus for further processing, and at step 1012 a treated cover gasstream may also be output from the separation apparatus.

Optionally, at step 1014 at least a portion treated cover gas stream canbe further processed or treated using a suitable gas treatmentapparatus.

At optional step 1016, at least a portion of the treated cover gas canbe recycled by reintroducing at least a portion of the treated cover gasstream into the reactor.

Optionally, some or all of steps 1000 to 1016 can be on-line stepsperformed while the heavy water nuclear reactor is operating.Alternatively, some or all of steps 1000 to 1016 can be off-line stepsperformed while the reactor is not operating.

The step of separating the ³He gas from the outlet gas stream mayinclude utilizing any suitable apparatus and/or carrying out anysuitable process including, for example, at least one of a thermaldiffusion process, a fractional diffusion process, a heat flush process,a superleak process and a differential absorption process.

While heavy water reactors including both a heavy water moderator andheavy water coolant are illustrated, the ³He extraction apparatuses andmethods described herein may be used on any suitable heavy waterreactor, including, for example, reactors having a heavy water moderatorand a non-heavy water coolant, reactors having a non-heavy watermoderator and a heavy water coolant and reactors having a non-heavywater moderator, a non-heavy water coolant but that include some othertype of heavy water circuit or system that is provided within thereactor such that ³He is formed in the heavy water system.

What has been described above has been intended to be illustrative ofthe invention and non-limiting and it will be understood by personsskilled in the art that other variants and modifications may be madewithout departing from the scope of the invention as defined in theclaims appended hereto.

The invention claimed is:
 1. A method of collecting ³He from a nuclearreactor, the method comprising: a. providing heavy water at least partof which is exposed to a neutron flux of the reactor; b. providing acover gas in fluid communication with the heavy water; c. operating thenuclear reactor whereby thermal neutron activation of deuterium in theheavy water produces tritium (³H) and at least some of the tritiumproduces ³He gas by β⁻decay and at least a portion of the ³He gasescapes from the heavy water and mixes with the cover gas; d. extractingan outlet gas stream, the outlet gas stream comprising a mixture of thecover gas and the ³He gas; and e. separating the ³He gas from the outletgas stream using at least one of a thermal diffusion process, afractional diffusion process, a heat flush process, a superleak processand a differential absorption process.
 2. The method of claim 1, furthercomprising outputting a ³He gas stream for further processing.
 3. Themethod of claim 2, further comprising treating the outlet gas stream toprovide a treated cover gas stream.
 4. The method of claim 3, furthercomprising mixing at least a portion of the treated cover gas streaminto the cover gas in fluid communication with the heavy water.
 5. Themethod of claim 1, wherein the step of extracting the outlet gas streamis performed while nuclear reactor is operating.
 6. The method of claim5, wherein the outlet gas stream is extracted as a generally continuousstream while nuclear reactor is operating.
 7. The method of claim 1,wherein the step of separating the ³He gas from the outlet gas stream isan on-line process that is performed while the nuclear reactor isoperating.
 8. The method of claim 1, wherein when the cover gas contactsthe heavy water at a free surface interface.
 9. A method of collecting³He from a nuclear reactor, the method comprising: a. providing heavywater at least part of which is exposed to a neutron flux of thereactor; b. operating the nuclear reactor whereby thermal neutronactivation of deuterium in the heavy water produces tritium (³H) and atleast some of the tritium produces ³He gas by β⁻decay and at least aportion of the ³He gas escapes from the heavy water; c. extracting anoutlet gas stream including the ³He gas; and d, separating the ³He gasfrom any other gas in the outlet gas stream using at least one of athermal diffusion process, a fractional diffusion process, a heat flushprocess, a superleak process and a differential absorption process.